1. Field of the Invention
The present invention relates to a semiconductor laser oscillating at a wavelength in a visible region of green to yellow colors applied to a display or the like, and more particularly to a semiconductor laser with a stacked structure for improving operation characteristics of a semiconductor laser using a compound semiconductor comprising a group II element and a group VI element formed on an InP substrate.
2. Description of the Related Arts
Semiconductor devices emitting light in visible to ultraviolet regions, that is, semiconductor lasers or light emitting diodes are important semiconductor devices in the current society/industrial fields, for example, for use in application to light sources for use in optical information recording devices (compact discs (CDs), digital versatile discs (DVDs), Blu-ray discs (BDs)) and color display apparatuses, and to solid laser excitation, fabrication, sensors, measuring instruments, or medical use, or white lamps. Table 1 shows the outline of wavelength bands and semiconductor materials of optical devices in such application use.
TABLE 1SemiconductorDevicedevice emittingemittinglight with yellowDevice emittingDevice emittingblue lightto green colorsred lightinfrared light400 nm band500 nm band600 nm band780 nm, 808 nm,(esp. 400 to(esp. 635 to860 nm, 915 nm,480 nm)670 nm)980 nm bandsIII-V groupNo appropriateIII-V groupIII-V groupnitrogencompoundcompoundcompoundcompoundsemiconductorsemiconductorcontainingsemiconductorlaser capable ofcontainingAlGa(In)Ascontainingcontinuous lasingAlGaInPAlGaInN
As can be seen from Table 1, for semiconductor devices emitting light from yellow to green colors in a 500 nm band which is a wavelength band intermediate between red and blue colors, not only research and development but also development for materials have not yet been conducted sufficiently. Therefore, semiconductor devices, especially, semiconductor lasers in the wavelength band described above, which have performance sufficient for practical use, have not been achieved.
As semiconductors for use in optical devices, group II-VI semiconductors and group III-V compound semiconductors are useful. However, green-yellow semiconductor lasers have not yet been put to practical use so far. The main reason is that no sufficient reliability has been obtained.
One of the causes for deterioration of devices is multiplication of crystal defects which are macro-defects in a laser active layer portion. Kato et. al have endeavored to decrease stacking faults of ZnSe-based materials formed on a GaAs substrate by devising crystal growing conditions, trially manufactured a laser with no defects in the active layer region and observed device life up to 400 hrs (E. Kato, et al., Electron. Lett. 34, 282 (1998)). It was pointed out that the life was restricted by movement of micro-defects due to compressive strains and micro-defects such as nitrogen doping in p-cladding layers.
In recent years, studies have been started on group II-VI compound semiconductor materials containing beryllium for group II elements as the constituent materials for green-yellow semiconductor lasers (specification of Japanese Patent Laid-open No. H08-148765, WO97/018592, Japanese Patent Laid-open No. 2004-95922, A. Waag et al., Journal of Crystal Growth 184/185 (1998)1-10). The present inventors have trially manufactured a BeZnSeTe-based LED (light Emitting Diode) using beryllium-containing group II-VI semiconductor materials and confirmed a device life of 5000 hrs at a room temperature at a light emitting wavelength of 575 nm, and injection current density of 130 A/cm2 (Kishino et al., Phys. Stat. Sol., 6, (2004) 1477-1486, Hayami et al., Pretext of the 52th Society of Applied Physics Conference, 31p-ZN-6, Yuki Nakai et al., phys. stat. sol (a) 201, 12(2004) pp. 2708-2711). It is considered that the results show a progress toward the practical use of green-yellow semiconductor lasers.
It is considered that the improvement for the reliability in this system is attributable to that a lattice-matched crystal layer could be used by the use of an InP substrate and that crystals are strengthened by the introduction of beryllium to provide an effect of suppressing the multiplication of crystal defects and dislocations. Further, the present inventors have trially manufactured semiconductor lasers by the material constitution identical with that of the LED and confirmed laser oscillation at 77 K pulse driving (Kishino et al., Phys. Stat. Sol., 6, (2004) 1477-1486). However, this structure did not attain continuous oscillation at a room temperature which is necessary for practical use.